1. Field of the Invention
The present invention generally relates to reducing data access time in disk drives, and more particularly to an apparatus and method for reducing data access time during seek operations in disk drives by increasing a voltage supplied to a voice coil motor (VCM) of the disk drive.
2. Description of the Prior Art
In hard disk drives, data is stored on magnetic media disks in concentric data tracks, which are divided into groups of data sectors. Servo information including track number, sector number, and tracking information is recorded in radially continuous narrow wedges between the groups of data sectors. The disk drive includes an actuator assembly having a voice coil motor (VCM), an actuator arm extending from the VCM, and a transducer head disposed at the end of the actuator arm. One way of improving disk drive performance is to reduce data access time, including the time required to move the transducer head of the actuator assembly from a current data track to a selected target data track.
The disk drive performs a seek operation to move the transducer head from a present data track to a target data track. The disk drive includes a servo system employed to seek to the target data track and thereafter follow the target data track on the disk. The servo system controls the VCM so that the VCM swings the actuator and its attached transducer head to access the target data track. Therefore, if the torque developed (T.sub.d) by the VCM could be increased, a corresponding reduction in data access time could be achieved. T.sub.d is given by the following Equations I:
Equations I EQU T.sub.d =Kt*I.sub.coil EQU Kt=Ke(Kt in Nm/A; Ke in V/rad/sec) EQU Bemf=Ke*Vel.sub.motor (Vel.sub.motor in rad/sec) EQU V.sub.applied =V.sub.source -Bemf=I.sub.coil *R.sub.coil EQU I.sub.coil =(V.sub.source -Ke*Vel.sub.motor)/R.sub.coil
Where:
T.sub.d is the torque developed by the VCM; PA1 Kt is the Torque constant of the VCM; PA1 I.sub.coil is VCM coil current; PA1 Ke is the Voltage constant of the VCM i.e. the Bemf factor; PA1 Bemf is the back emf of the VCM; PA1 Vel.sub.motor is the velocity of the VCM; PA1 V.sub.applied is the voltage applied to the VCM coil; PA1 V.sub.source is the voltage of the VCM power source supplying power to the VCM; and PA1 R.sub.coil is the total resistance of the VCM coil from the VCM power source to ground
As can be shown by Equations I above, T.sub.d can be increased by increasing Kt and/or I.sub.coil. Unfortunately, increasing I.sub.coil causes a corresponding heating of the VCM coil, proportional to the square of the current, which can result in heat related breakdown of VCM components, including the VCM coil itself. In particular, seek operations require the highest current to be supplied to the VCM in order to achieve competitive access times. Conversely, increasing Kt equates to an equal increase in Ke which increases the Bemf of the VCM. Therefore, the Bemf approaches V.sub.source as Kt is increased. Consequently, if V.sub.source cannot be increased, less voltage V.sub.applied is available and I.sub.coil is reduced as Kt is increased thereby limiting the performance of the VCM. The V.sub.source supplied to the VCM in disk drives, however, is fixed because, for compatibility with host systems, a standard power supply having a fixed voltage is used to supply power to the VCM. This fixed voltage power supply to the VCM therefore limits how much effect the VCM can have on reducing access times.
For reasons stated above and for other reasons presented in greater detail in the detailed description of the present specification, there is a desire to reduce data access time during seek operations in disk drives. In particular, there is a need to improve VCM performance by increasing the torque constant of the VCM to achieve a corresponding decrease in data access time in seek operations in disk drives. It would also be desirable to increase the torque constant of the VCM to permit a reduction in VCM coil current to reduce I.sup.2 R power losses in the system while still maintaining a given torque in the VCM.